1. Field of the Invention
This invention relates to a drilling machine provided with an electromagnetic base, which possesses a function of self-operated control fulfilled by sequentially detecting a load exerted on a driving motor for an electric drill. More particularly, this invention relates to a drilling machine provided with an electromagnetic base, which is adapted to bore a hole in a workpiece by utilizing an annular cutter and serves to issue warnings and/or to operate a control system for the electric drill when an overload is brought about on the driving motor of the electric drill while the machine is in operation.
2. Description of the Prior Art
There has been hitherto proposed and actually used a drilling machine comprising an electromagnetic base capable of electromagnetical fixation on a workpiece and an electric drill integrally mounted on the electromagnetic base. The driling machine of this type excels in workability, whereas the electromagnetic base will readily fail to be fixed stably on the workpiece even when a slightly large load is brought about on the electric drill. Particularly, in a case where an annular cutter requiring larger torque compared with a twist drill is used for this drilling machine, the machine is susceptible to accidents such as side slipping of the machine on the workpiece, breakage of the annular cutter and seizure of the driving motor. To remedy such disadvantages of the use of the annular cutter, there has been proposed a drilling machine comprising a punching assembly disposed behind the elctromagnetic base and serving as a means for securely fixing the machine on a workpiece and a control switch which is actuated upon detection of abnormal motion of the electromagnetic base thereby bringing the driving motor to a stop. (U.S. Pat. Nos. 3,969,036 and 4,261,673) Such a drilling machine has, however, entailed a disadvantage that the control switch for the driving motor is actuated when an accident has already been brought about, and therefore, it fails to serve as a measure for completely preventing these accidents as described above.